Oni diris, ke la vojo denove estos sekura morgaŭ, se ne okazos alia akcidento.

Oni is the Esperanto indefinite subject, roughly like English people, someone, they, or one in a general sense.

So Oni diris means something like:

• People said
• They said
• It was said

It does not refer to a specific group. It is useful when the speaker either does not know or does not want to name who did the action.

Because diris refers to the time of the saying, not the time of the road being safe.

The sentence has two time layers:

• oni diris = someone said it in the past
• estos sekura morgaŭ = the road will be safe tomorrow

So Esperanto handles this just like English:

• They said that the road would/will be safe tomorrow

The past tense in diris does not force the rest of the sentence to be past too. Each verb shows its own time.

Ke means that and introduces a content clause.

In this sentence:

• Oni diris, ke... = They said that...

It connects the main clause to what was said.

Esperanto uses ke very regularly for this kind of clause, often more consistently than English, where that is sometimes omitted.

So while English can say:

• They said the road would be safe

Esperanto normally keeps ke:

• Oni diris, ke la vojo...

Both are future-tense verbs, marked by -os.

• estos = will be
  from esti = to be
• okazos = will happen / will occur
  from okazi = to happen

So:

• la vojo denove estos sekura morgaŭ = the road will be safe again tomorrow
• se ne okazos alia akcidento = if another accident does not happen

Esperanto tense endings are very regular:

• -as present
• -is past
• -os future
• -us conditional
• -u imperative/jussive
• -i infinitive

Because sekura is an adjective describing la vojo.

• sekura = safe (adjective)
• sekure = safely (adverb)

Here the sentence says the road will be safe, so we need an adjective.

Compare:

• La vojo estas sekura. = The road is safe.
• Oni veturas sekure. = One drives safely.

After esti, Esperanto normally uses an adjective when describing a noun.

Because it agrees with la vojo, which is:

• singular
• not accusative

So the correct form is sekura.

Agreement in Esperanto works like this:

• singular adjective: sekura
• plural adjective: sekuraj
• accusative singular: sekuran
• accusative plural: sekurajn

Here:

• la vojo = the road
  singular, subject
• therefore sekura = singular, not accusative

If it were plural, you would have:

• la vojoj estos sekuraj

Because alia akcidento is the subject of okazos, not a direct object.

In the clause:

• se ne okazos alia akcidento

the thing that happens is another accident. So alia akcidento is the subject.

A useful way to think of it:

• Another accident will happen
  = Alia akcidento okazos

Since it is not receiving the action of a transitive verb, it does not take -n.

Denove means again.

Here it means the road will be safe again, implying it was safe before, then became unsafe, and is expected to return to a safe state.

Its position is fairly flexible, but where it appears can slightly affect what it seems to modify most directly.

In this sentence:

• la vojo denove estos sekura morgaŭ

the natural meaning is that the road will once again be safe.

Other placements are possible, for example:

• La vojo estos denove sekura morgaŭ
• Morgaŭ la vojo denove estos sekura

These are still understandable, with only slight differences in emphasis.

Yes, morgaŭ can often move.

Esperanto word order is fairly flexible because the grammar is marked clearly by endings and particles. Morgaŭ is an adverb of time, so it can appear in different places depending on emphasis.

All of these are possible:

• Oni diris, ke la vojo denove estos sekura morgaŭ...
• Oni diris, ke morgaŭ la vojo denove estos sekura...
• Oni diris, ke la vojo morgaŭ denove estos sekura...

The original version is perfectly natural. Placing morgaŭ later keeps the flow smooth and puts the time expression close to the future statement.

Because ne usually goes directly before the word or phrase it negates.

Here:

• ne okazos = will not happen

So:

• se ne okazos alia akcidento = if another accident does not happen

That is the most neutral and common placement.

You may also see different word orders in Esperanto, but putting ne right before the verb is usually the clearest choice when negating the whole action.

Because Esperanto normally uses se for if-clauses, just as English does.

So:

• se ne okazos alia akcidento = if another accident does not happen

The verb stays in the future because the clause refers to a future possibility.

Esperanto does have the conditional ending -us, but that is used for things like would, not for every if-clause automatically.

For example:

• La vojo estus sekura, se... = The road would be safe, if...

But in your sentence, the speaker presents the future as expected or predicted, not as a purely hypothetical conditional result, so estos and okazos are natural.

Yes. In many contexts, oni diris is a very natural match for English they said when they means people in general, not a specific group.

For example:

• Oni diras, ke... = They say that...
• Oni diris, ke... = They said that...

But remember that oni is more explicitly indefinite than English they. It means the speaker is not identifying the source.

So depending on context, English translations could include:

• they said
• people said
• someone said
• it was said

All can fit, even though Esperanto uses the single word oni.